1. Field of the Invention
The present invention is related to the improvement in oxidation stability of lubricants and, more particularly, to the oxidation stability of lubricating oils by a combination of at least two antioxidants.
2. Description of Related Art
Lubricating oils, as used in the internal combustion engines of automobiles, trucks, trains, ships, and aviation turbine oils of jet aircraft, are subjected to a demanding environment during use. This environment results in the oxidation of the oil, which oxidation is catalyzed by impurities that are present in the oil, e.g., iron compounds, and further promoted by the elevated temperatures that arise during use. This oxidation of lubricating oils during use is usually controlled, at least to some extent, by the addition of antioxidants that may extend the useful life of the oil.
U.S. Pat. No. 5,523,007 discloses a lubricant oil composition comprising a diesel engine lubricating oil and, as antioxidant, a compound of the formula:
wherein R1 and R2 are each independently of the other C1-C12 alkyl and X is
or —CH2—S—R and R is a straight chain or branched alkyl radical of the formula —CnH2n+1, wherein n is an integer from 8 to 22.
U.S. Pat. No. 6,559,105 discloses a composition of an antioxidant of the formula:
where R3 is an alkyl group of 2 to 6 carbon atoms, and a dispersant or a detergent, is a useful additive package for lubricant compositions.
The disclosures of the foregoing are incorporated herein by reference in their entirety.